Hydrostatic axial piston machine

ABSTRACT

The invention relates to a hydrostatic axial piston engine with a cylinder drum ( 106 ) which is mounted rotatably in a housing ( 103 ). Longitudinally displaceable working pistons ( 111 ) are disposed in cylinder boreholes ( 110 ) in the cylinder drum ( 106 ). The working pistons ( 111 ) are connected movably to sliding blocks ( 149 ) and are supported on a face ( 116 ) of an inclined disk ( 115 ). Furthermore, the hydrostatic axial piston engine ( 101 ) has a control piston ( 126 ) which interacts with the inclined disk ( 115 ) for adjusting an angle of inclination of the face ( 116 ) with respect to an axis of rotation ( 144 ) in the cylinder drum ( 106 ). The control piston ( 126 ) is supported over a control piston sliding block ( 149 ′) on the face ( 116 ) of the inclined disk ( 115 ) in order to exert a regulating force on the inclined disk ( 115 ).

The invention relates to a hydrostatic axial piston machine having the features of the preamble of claim 1.

In axial piston machines a cylinder drum is disposed in a rotatable manner. The cylinder drum is connected in a rotationally fixed manner to a driving shaft. A plurality of cylinder bores are arranged distributed along a circumferential circle in the cylinder drum. In each of the cylinder bores a working piston is disposed in a longitudinally displaceable manner. In order during a rotation of the cylinder drum to achieve a stroke of the working pistons disposed in the cylinder bores, the working pistons are supported on a contact surface of a swash plate. The contact surface is disposed obliquely relative to the axis of rotation of the cylinder drum.

In adjustable axial piston machines the inclination of this contact surface is adjustable. For this purpose, the contact surface is disposed on a swash plate that frequently takes the form of a swiveling cradle. Such an axial piston machine is known from DE 199 49 169 A1. In order to be able to adjust the angle of inclination of the contact surface relative to the axis of rotation of the axial piston machine, an adjusting device is disposed there, in a housing opening. The adjusting device comprises a positioning piston, which for adjusting the inclination of the swash plate interacts with the swash plate. For this purpose, there is formed on the positioning piston a spherical head that engages into a spherical recess of the swash plate. The articulated connection thus formed between the positioning piston and the swash plate enables an inclination of the two elements relative to one another, such as arises automatically during an adjustment of the angle of the swash plate. The location opening that is provided for receiving the adjusting device in the housing of the axial piston machine is disposed obliquely relative to the axis of rotation of the cylinder drum.

In the axial piston machine known from DE 199 49 169 A1 it is disadvantageous that a special articulated connection of the swash plate to the positioning piston of the adjusting device is formed. The fixed distance of the working point of the positioning piston on the swash plate moreover necessitates a lateral motion compensation, in the event of a change of inclination of the swash plate. This increases the complexity of manufacture of the axial piston machine considerably. What is more, the oblique arrangement of the adjusting device in a recess of the housing is a drawback because the arising forces that are necessary for adjustment of the swash plate are considerable. The load removal therefore has to be effected via the housing, thereby entailing the use of a high-quality housing material. This increases the cost of the whole axial piston machine.

The object of the present invention is therefore to provide an axial piston machine that allows an improved adjustment of the angle of inclination of the swash plate and is simple and economical to manufacture.

The object is achieved by the axial piston machine according to the invention having the features of claim 1.

The axial piston machine according to the invention has a housing, in which a cylinder drum is rotatably mounted. Disposed in the cylinder drum is a plurality of cylinder recesses, in each of which a working piston is disposed in a longitudinally displaceable manner. The working pistons are supported in each case by means of a sliding shoe against a contact surface of a swash plate. The connection between the working piston and the sliding shoes is of a movable design. A positioning piston is further provided, which interacts with the swash plate in order to adjust an angle of inclination of the contact surface relative to the axis of rotation of the cylinder drum. According to the invention, the positioning piston is connected to a positioning-piston sliding shoe, which is supported on the contact surface of the swash plate in order in this way to generate a positioning force upon the swash plate. The design according to the invention has the advantage that, as far as the swash plate is concerned, only a contact surface need be formed. This contact surface generally takes the form of a simple flat surface. Thus, by means of the sliding shoe a thrust force may easily be exerted by the positioning piston on the contact surface of the swash plate. At the same time, during a positioning movement the support point of the positioning-piston sliding shoe may automatically adapt in radial direction on the contact surface. A special construction of an articulated connection between the swash plate and the positioning piston is not necessary.

Advantageous developments of the axial piston machine according to the invention are outlined in the sub-claims.

Preferably, a ball joint is disposed between the positioning piston and the positioning-piston sliding shoe. The effect achieved by connecting the positioning piston to the positioning-piston sliding shoe by a ball joint is that the assembly position of the positioning piston and/or of the positioning-piston sliding shoe with regard to a twisting of the positioning piston or of the positioning-piston sliding shoe is insignificant. In this case, it is particularly advantageous if the positioning piston and the working pistons have an identical geometry. A separate manufacture of the positioning piston therefore no longer applies. As a result of the increased number of identical parts inside the axial piston machine it is therefore possible to achieve a considerable cost saving. It is in particular also advantageous for the positioning-piston sliding shoe to be designed identically with the sliding shoes of the working pistons. It is particularly preferred if both the positioning piston is identical with the working pistons and the positioning-piston sliding shoe has an identical geometry to the sliding shoes of the working pistons.

It is further preferred if the positioning piston is disposed in a positioning-device housing and is displaceable there along a longitudinal axis of the positioning-device housing. The longitudinal axis of the positioning-device housing extends at least approximately parallel to an axis of rotation of the cylinder drum. Such an approximately parallel arrangement reduces the required installation space in lateral direction relative to the axis of rotation of the cylinder drum. In the positioning-device housing a stepped recess is preferably disposed. Disposed in axial succession in the stepped recess are the positioning piston as well as a valve piston of a valve for activating a positioning pressure that acts upon the positioning piston. In the positioning-device housing it is therefore possible to provide a single multi-stepped bore, in which all of the components needed to generate a positioning movement of the swash plate and hence a working volume of the axial piston machine are disposed. In particular, there may also be provided inside the positioning-device housing a feedback spring, with the aid of which the position of the positioning piston is fed back to the valve piston. Such feedback enables an adjustment of the displacement volume of the hydrostatic piston machine that corresponds to, for example is proportional, to a control signal.

In this case, it is particularly preferred if the feedback spring is identical with press-on springs inserted in the cylinder bores of the cylinder drum. The use of the, in any case necessary, press-on springs also in the adjusting device means that the number of identical parts inside the axial piston machine is again increased and so a cost reduction is achieved.

It is further preferred if the positioning-device housing is fastened in a port plate of the axial piston machine. Disposed in the port plate are the pressure lines for supplying and removing the pressure medium delivered for example by means of a hydraulic pump. If therefore the positioning device housing is disposed in the port plate, then the valve piston disposed in the positioning device housing may be loaded with the pressures required for adjustment via shorter lines and/or channels. It is possible to dispense with external lines because all of the channels may be provided inside the port plate and/or inside the positioning device housing. In particular, assembly is also facilitated because the port plate may be preassembled jointly with the adjusting device as an assembly group and then inserted, after insertion of the driving gear, into the housing of the axial piston machine.

An embodiment according to the invention of an axial piston machine is represented in the drawings and explained in detail in the following description. The drawings show:

FIG. 1 a diagrammatic representation of a hydrostatic axial piston machine with a known adjusting device;

FIG. 2 a hydraulic circuit diagram for explaining the mode of operation of the adjusting device; and

FIG. 3 an enlarged representation of an axial piston machine according to the invention.

First, before an axial piston machine according to the invention is described in detail with reference to FIG. 3, the basic layout and the function of a hydrostatic axial piston machine will be described with reference to FIG. 1.

In FIG. 1 a hydrostatic axial piston machine 1 is represented. The known hydrostatic axial piston machine 1 comprises a driving gear 2, which is disposed in a housing 3. For inserting the driving gear 2 into the housing 3, the housing 3 is open at one end. After assembly of the driving gear 2 in the housing 3, the open end is closed by means of a port plate 4. On the port plate 4 the line connections are provided in a manner that is not represented.

The driving gear 2 comprises a driving shaft 5 and a cylinder drum 6 connected thereto in a rotationally fixed manner. The driving shaft 5 is disposed so as to be rotatable jointly with the cylinder drum 6 in the housing 3.

For this purpose, the driving shaft 5 at one end of the housing 3 is mounted rotatably in a first bearing 7. At the opposite end of the driving shaft 5 a second bearing 8 is provided, which in the illustrated embodiment is disposed in the port plate 4. The driving shaft 5 with one end 9 penetrates the first bearing 7 as well as the end face of the housing 3 of the hydrostatic axial piston machine 1.

For the following description it is assumed that the hydrostatic axial piston machine 1 is a variable displacement hydraulic pump 1. The end 9 of the driving shaft 5 is therefore connected to a drive motor for driving the hydraulic pump.

In the cylinder drum 6 a plurality of cylinder bores 10 are introduced into the cylinder drum 6 and arranged distributed along a circumferential circle. Disposed in each cylinder bore 6 is a working piston 11. The working piston 11 may be displaced in longitudinal direction along the centre line of the cylinder bore 10. The working piston 11 is connected by a ball joint connection 13 movably to a sliding shoe 12. The sliding shoes 12 of the working pistons 11 are supported by a sliding face on a awash plate 15. The swash plate 15 in the illustrated embodiment takes the form of a swiveling cradle that is disposed rotatably in a spherical bearing. On the side of the swash plate 15 facing the cylinder drum 6 is a contact surface 16 in the form of a flat surface.

Whereas in FIG. 1 the contact surface 16 and/or the swash plate 15 is represented in its neutral position, which corresponds to a zero delivery volume of the hydraulic pump, the working pistons 11 are shown in a position that would correspond to a swivel angle α of the swash plate 15.

During a rotation of the driving shaft 5 the cylinder drum 6, because of the rotationally fixed connection, also rotates. In this case, the sliding shoes 12 are supported against the contact surface 16 of the swash plate 15 and constrain the working pistons 11 into a reciprocating movement. In order during an intake stroke to prevent the sliding shoes 12 from lifting off the contact surface 16 of the swash plate 15, a retraction plate 14 is provided. The retraction plate 14 follows the angle of inclination of the swash plate 15 and is mounted on a spherical bearing 17.

For temporarily connecting the cylinder bores 10 to the lines of a hydrostatic circuit a control plate 18 is provided. Formed in the control plate 18 are control openings 19, 20, with which during a revolution of the cylinder drum 6 the cylinder bores 10 alternately communicate. In order to hold the cylinder drum 6 at the mouth side of the cylinder bores 10 in sealing abutment with the control plate 18, a spring 21 is provided in the interior of the cylinder drum 6. The spring 21 is supported on the one hand against the cylinder drum 6, in which for this purpose for example a Seeger circlip ring is disposed as a first spring bearing. A second spring bearing is formed at the opposite end of the spring 21 on the driving shaft 5.

For adjusting the working volume of the axial piston machine 1 an adjusting device 22 is provided. The adjusting device 22 is actuated by means of a proportional magnet 23. The proportional magnet 23 acts in a non-illustrated manner upon a valve piston of the adjusting device 22 that adjusts a positioning pressure, which acts upon a positioning piston 26.

Formed on the positioning piston 26 is a spherical connection element 24. This spherical connection element 24 engages into a spherical recess 25 disposed in the swash plate 15. The longitudinal axis of the adjusting device 22 with the axis of rotation of the cylinder drum 6 includes an angle differing from 0.

FIG. 2 shows a hydraulic circuit that is provided for adjusting an axial piston machine 1. The hydrostatic axial piston machine 1, which in the case of a hydraulic pump is driven via the driving shaft 5, takes in pressure medium through a suction line 27 from a tank volume 28. In the illustrated embodiment, an arrangement in an open circuit is shown. The axial piston machine may however also be arranged in a closed circuit. The pressure medium taken in by the hydrostatic axial piston machine 1 is delivered in accordance with the adjusted delivery volume into a working line 29. For adjusting the delivery volume of the hydraulic pump the adjusting device 22 is provided. The adjusting device 22 comprises, besides the positioning piston 26, a positioning-pressure regulating valve 32. The positioning-pressure regulating valve 32 adjusts a positioning pressure that acts upon the positioning piston 26. The positioning pressure acting upon the positioning piston 26 is removed from the working line 29 through a removal line 31. The positioning piston 26 is loaded in one direction with a spring force by means of a restoring spring 33. In a positioning pressure chamber 35 a feedback spring 34 is disposed, which transmits to a valve piston of the positioning-pressure regulating valve 32 a force that is dependent upon the position of the positioning piston 26. The positioning-pressure regulating valve 32 in its position represented in FIG. 2 is situated in the normal position of the positioning device 22. In this position the removal line 31 is connected to a positioning pressure line 36. As a result, the pressure prevailing in the working line 29 arises in the positioning pressure chamber 35. This pressure acts upon the positioning piston 26 with its piston area oriented towards the positioning pressure chamber pressure space 35. The positioning piston 26 is consequently deflected in FIG. 2 to the left. The positioning piston 26 owing to the positioning movement compresses the restoring spring 33. Upon a positioning movement of the positioning piston 26 counter to the action of the restoring spring 33, the hydrostatic axial piston machine 1 is adjusted in the direction of its minimum delivery volume.

From the indicated normal position of the positioning-pressure regulating valve 32 the positioning-pressure regulating valve 32 may be loaded with a force in the direction of a second end position. This force is generated for example by a proportional magnet 23. The force of the proportional magnet 23 acts counter to the action of the feedback spring 34. If the proportional magnet 33 then receives a control signal, the valve piston of the control-pressure regulating valve 33 experiences a force in the direction of its second end position. In this second end position the positioning pressure line 36 is connected to a connection line 37. The positioning-pressure regulating valve 32 is infinitely adjustable between these two end positions. During normal operation the connection line 37 is connected by a safety valve 39 to the tank line 30. In the second end position of the positioning-pressure regulating valve 32, therefore, the positioning pressure chamber 35 is connected by the connection line 37 to the tank line 30 and the positioning pressure chamber 35 is relieved into the tank volume 28. As a result of this, the force upon the piston area of the positioning piston 26 reduces and the restoring spring 33 moves the positioning piston 26 in such a way that the hydrostatic axial piston machine 1 is adjusted in the direction of an increasing delivery volume.

In order to be able to adjust a proportional positioning movement and/or a proportional delivery volume of the hydrostatic axial piston machine 1, the force of the feedback spring 34 acts in the opposite direction to the force of the proportional magnet 23 upon the valve piston of the positioning-pressure regulating valve 32. Thus, there is exerted on the valve piston of the positioning-pressure regulating valve 32 a force that is dependent upon the respective position of the positioning piston 26.

The positioning pressure line 36 is further connected by a bypass line 36′ to the connection line 37. In the bypass line 36′ a throttle point 38 is disposed. Via the throttle point 38 a flow of pressure medium out of the positioning pressure chamber 35 therefore becomes possible if for example the positioning-pressure regulating valve 32 is adjusted from its first end position slightly in the direction of its second end position as a result of the proportional magnet 23 receiving a lower control signal.

The safety valve 39 in its normal position connects the connection line 27 in the previously described manner to the tank line 30. This normal position is defined by means of a safety valve spring 40. In the opposite position of the safety valve 39, however, a further connection line 41 is connected to the connection line 37. The further connection line 41 branches off from the removal line 31. In the opposite position of the safety valve 39, therefore, the pressure of the working line 29 is supplied to the connection line 37. The pressure prevailing in the further connection line 41 is further supplied via a first measuring line 42 to a measuring area formed on the safety valve 39. The hydraulic force effective at the measuring area acts counter to the action of the safety valve spring 40. Conversely, the pressure in the tank line 30 acts via a second measuring line 43 in the same direction as the safety valve spring 40 upon the safety valve 39.

By means of the safety valve spring 40, which is designed as an adjustable spring, the opening differential pressure of the safety valve 39 may be adjusted. If this opening pressure is exceeded by the pressure difference between the pressures supplied via the first measuring line 42 and the second measuring line 43, then the safety valve 39 is adjusted in the direction of its second end position. With increasing adjustment in the direction of the second end position, the pressure in the positioning pressure chamber 35 rises, even if the positioning-pressure regulating valve 32 is situated in its second end position. Consequently, the positioning piston 26 is adjusted in FIG. 2 to the left, thereby leading to a reduction of the delivery volume.

FIG. 3 shows an enlarged representation of a detail of an axial piston machine 101 according to the invention. For the sake of greater clarity, the as such known elements of the axial piston machine 101 are not represented. Instead, the axial piston machine 101 in the region of the adjusting device 122 is represented to an enlarged scale. Elements and features that are identical to FIG. 1 are provided with reference characters increased by 100.

As already explained with reference to FIG. 1, in the cylinder drum 126, in cylinder bores 110 provided there, working pistons 111 are disposed in a longitudinally displaceable manner. The working pistons 111 are connected by means of a ball joint connection 113 to sliding shoes 112. For this purpose, a spherical head 147 is formed on an end of the working pistons 111 that projects from the cylinder bores 110 of the cylinder drum 106. This spherical head 147 engages into a spherical recess 148 of the sliding shoe 149. Introduced from the opposite side into the working pistons 111 is a recess 146. The recess 146 preferably takes the form of a bore, and is so dimensioned that a press-on spring 145 may be disposed therein. A lubricating oil bore 150 connects the recess 146 to the head end of the working piston 111. The press-on spring 145 in the non-tensioned state is longer than the maximum distance between the head end of the bore 146 and the opposite end of the cylinder bore 110. The press-on spring 145 therefore exerts a force both on the cylinder drum 106 and on the working piston 111. The cylinder drum 106 is therefore held in abutment with the control plate 118. The working piston 111 on the other hand is held jointly with the sliding shoe 149 in abutment with the contact surface 116 of the swash plate 115.

A sliding face 152 is formed on the sliding shoe 149. The sliding face 152 preferably has at least one lubricating oil groove 151. The lubricating oil groove is connected by a connection bore to the region in the spherical recess 148. The pressurized pressure medium situated in the cylinder bore 110 is therefore conveyed through the lubricating oil bore 150 and the connection bore in the sliding shoe 149 to the lubricating oil groove 150, where it effects a hydrostatic relief of the sliding shoe 149. The contact area between the spherical recess 148 and the spherical head 147 is further supplied with pressure medium for lubrication purposes. Because of the ball-joint-like connection between the sliding shoe 149 and the spherical head 147 of the working piston 111 the inclination of the sliding face 147 relative to the longitudinal axis of the working piston 111 may be varied. It is thereby ensured that the sliding face 152 may be adapted in every possible angular position of the contact surface 147 relative to the axis of rotation 144 of the cylinder drum 106. The spherical recess 148 encompasses the spherical head 147 to such an extent that tensile forces may also be transmitted between the working piston 111 and the sliding shoe 149.

For adjusting the angle of the contact surface 116 relative to the axis of rotation 144 of the cylinder drum 106 the positioning device 122 is provided.

For generating a positioning movement the positioning device 122 comprises a positioning piston 126. The positioning piston 126 is of an identical design to the working piston 111. A repeat description of the individual components with regard to the positioning piston 126 is therefore dispensed with. The positioning piston 126 is connected by a joint connection 113′ to a positioning-piston sliding shoe 149′. The positioning-piston sliding shoe 149′ corresponds in its construction to the sliding shoe 149 already described in connection with the working piston 111.

The positioning-piston sliding shoe 149′ therefore also has a sliding face 152′ that is situated in abutment with the contact surface 116. A positioning force that is exerted by the positioning piston in FIG. 3 to the left may therefore be transmitted to the contact surface 116 of the swash plate 115. In the recess 146 of the positioning piston 126 a spring is likewise disposed. This spring takes the form of a feedback spring 145′. The feedback spring 1451 is supported on the one hand against the head end of the recess 146′ in the positioning piston 126. The opposite end of the feedback spring 145′ is supported against a spring bearing 147. The spring bearing 147 in turn abuts a first end of a valve piston 158. There is therefore exerted on the valve piston 158 a force that depends both upon the position of the valve piston 158 and upon the position of the positioning piston 126. Thus, a backward-driving force is exerted as positional information on the positioning piston 158, as already described with reference to the hydraulic circuit diagram of FIG. 2. This force is dependent upon the position of the swash plate 115 and hence upon the adjusted delivery volume.

The positioning device 122 comprises a positioning device housing 153. The positioning device housing 153 is inserted, preferably screwed, into a recess in the port plate 104. The port plate 104 closes the housing 103 of the hydrostatic axial piston machine 101.

The positioning device housing 153 has a stepped recess 154. The stepped recess 154 takes the form of a bore and penetrates the positioning device housing 153 along a longitudinal axis 162 of the positioning device housing 153. The longitudinal axis 162 of the positioning device housing 163 is preferably oriented parallel to the axis of rotation 144 of the cylinder drum 106. However, depending on the precise structural design of the port plate 104 a slight deviation of a few degrees from parallel may also arise.

In the end of the positioning device housing 153 facing the swash plate 115 a liner 155 is inserted. The liner 155 is of a substantially pot-shaped design, wherein in the base of the liner 155 a through-opening is provided. The through-opening of the liner 155 is disposed approximately in the region of the first end of the valve piston 158 and so dimensioned that the spring bearing 157 may pass through.

The liner 145 is retained by means of a Seeger circlip ring 160 in the swash-plate end of the adjusting device housing 122. The positioning piston 126 and the valve piston 158 are arranged axially offset relative to one another in the positioning device housing 153. The positioning piston 126 is workingly connected by the feedback spring 145′ to the valve piston 158. On the valve piston 158 annular grooves 159, 160 are formed. By means of the annular grooves 159, 160 a connection between a removal channel 131 and the positioning pressure chamber 135 may be established in dependence upon the position of the valve piston 158 relative to the positioning device housing 153. In dependence upon the position of the valve piston 158 the annular groove 160 connects the removal channel 131 to the port 161 covered in FIG. 3 by the valve piston 158. The port 161 is connected by a likewise non-illustrated channel to the positioning pressure chamber 135. Upon loading of the valve piston 158 with an axial force by the proportional magnet 123, however, the second annular groove 159 is brought into a position, in which the port 161 is connected to the connection channel 137 in a manner allowing throughflow. The connection channel 137 is formed in the port plate 104.

The axial force that is generated by a proportional magnet 123 is transmitted by means of a tappet 159 to the end of the valve piston 158 remote from the spring bearing 157. In the region of the end of the valve piston 158 remote from the spring bearing 157 a normal position spring 161 is disposed. The normal position spring 161 is supported on the one hand against the adjusting device housing 163 and on the other hand against a collar 165.

By virtue of designing the adjusting device 122 using parts that may be used both as working piston 111 and as sliding shoe 149, a considerable simplification of the hydrostatic axial piston machine 101 is achieved. In particular, manufacture is made more economical as a separate parts manufacture for the positioning piston may be avoided. In particular, manufacture of the swash plate 115 is also extremely simplified. In contrast to the axial piston machine of prior art, all that is required is to produce a uniformly flat contact surface 116 that extends over the entire circumference of the swash plate 115. Not only is the plurality of working pistons 111 supported via their sliding shoes 149 on this contact surface 116 but a positioning force may also be transmitted from the positioning piston 126 by means of the positioning-piston sliding shoe 149′ to the contact surface 116. To enable a resetting of the swash plate 115, a restoring spring 133 is disposed at the side of the swash plate 115 remote from the contact surface 116. The restoring spring 133 loads the swash plate 115 with a restoring force counter to the positioning force of the positioning piston 126.

If the pressure chamber 135 of the positioning device 122, given a corresponding position of the valve piston 128, is therefore relieved into the tank volume 28, then because of the force of the restoring spring 133 the swash plate 115 is brought back into its neutral position. In this neutral position the hydrostatic axial piston machine 101 is adjusted to its maximum delivery volume. Thus, given an unpressurized system because of the neutral position of the hydrostatic axial piston machine 101 first a pressure build-up in the working line 29 may occur. This built-up pressure is then supplied, because of the normal position of the valve piston 158, also to the pressure chamber 135. As a result of this, an equilibrium of forces arises at the valve piston 158. The equilibrium of forces exists between the force of the feedback spring 145 acting upon the valve piston 158 and the equidirectional force of the normal position spring 163 as well as the oppositely directed force of the proportional magnet 123, which acts via the tappet 159 upon the valve piston 158.

The supplying of the sliding face 152′ of the positioning-piston sliding shoe 159′ is effected likewise in a comparable manner to the previously described lubrication of the sliding face 152 of the sliding shoe 149. For this purpose, the sliding face 152′ is connected by a connection bore as well as a lubricating oil bore in the positioning piston 126 to the positioning pressure chamber 135. Thus, from the pressure acting in the positioning pressure chamber 135 a lubricating film is generated both between the spherical head of the positioning piston 126 and at the sliding face 152′. An exact swiveling angle adjustment is achieved as a result of the reduction of friction.

The invention is not limited to the represented embodiment. The axial piston machine may therefore also take the form of a hydraulic motor and in particular individual features of the invention may also be combined with one another. 

1. Hydrostatic axial piston machine having a cylinder drum, which is mounted rotatably in a housing and in which working pistons are disposed in a longitudinally displaceable manner in cylinder bores, wherein the working pistons are movably connected to sliding shoes and the working pistons are supported via the sliding shoes against a contact surface of a swash plate, and having a positioning piston, which interacts with the swash plate in order to adjust an angle of inclination of the contact surface relative to an axis of rotation of the cylinder drum, wherein the positioning piston is connected to a positioning-piston sliding shoe and via the positioning-piston sliding shoe the contact surface of the swash plate is loaded with a positioning force.
 2. Hydrostatic axial piston machine according to claim 1, wherein the positioning piston and the positioning-piston sliding shoe are connected to one another by a ball joint.
 3. Hydrostatic axial piston machine according to claim 1, wherein the positioning piston and the working pistons have an identical geometry.
 4. Hydrostatic axial piston machine according to claim 1, wherein the positioning-piston sliding shoe and the sliding shoes of the working pistons have an identical geometry.
 5. Hydrostatic axial piston machine according to claim 1, wherein the positioning piston is disposed in a positioning device housing in a longitudinally displaceable manner along a longitudinal axis of the positioning device housing and the longitudinal axis of the positioning device housing extends at least approximately parallel to the axis of rotation of the cylinder drum.
 6. Hydrostatic axial piston machine according to claim 5, wherein the positioning piston is disposed in a stepped recess of the positioning device housing and a valve piston is arranged offset in axial direction relative to the positioning piston in the stepped recess.
 7. Hydrostatic axial piston machine according to claim 5, wherein for feedback of positional information of the positioning piston a feedback spring is provided, which is supported on the one hand against the positioning piston and on the other hand against a valve piston.
 8. Hydrostatic axial piston machine according to claim 7, wherein the feedback spring is identical with press-on springs inserted in the cylinder recesses.
 9. Hydrostatic axial piston machine according to claim 1, wherein the positioning device housing is fastened in a port plate of the axial piston machine. 